The invention relates to a projection exposure system for microlithography. These types of projection exposure system generally have a mask holding device in the form of a mask table or of a so-called “reticle stage” for holding a mask or a so-called “reticle” with mask structures disposed on the latter. This type of projection exposure system generally further comprises a substrate holding device in the form of a so-called “wafer stage” for holding a substrate in the form of a wafer and projection optics for imaging the mask structures onto the substrate.
With conventional projection exposure systems the quality of the image often suffers from blurring. If the image position drifts over the wafer while exposing a field, the latent image is blurred in the photoresist. The effect of this is overlay errors in the printed structures. These blurring problems occur to a particularly large extent with EUV projection exposure systems. EUV projection exposure systems expose structures with light with a wavelength in the extreme ultraviolet wavelength range, e.g. with a wavelength of 13.5 nm. In the roadmap of the semiconductor industry optical lithography in the EUV plays a key role. Only mirrors are considered as optical components here. With mirror optics a change to the mirror position and/or the mirror tilt position leads first and foremost to a shift in the image. The requirements for the mechanical stability of the optical components are clearly tightened in comparison to refractive systems.
With conventional projection exposure systems the field position while exposing a wafer is controlled a number of times with appropriate adjustment and so-called “alignment” sensors, and appropriate corrective measures are introduced. For this purpose the actual exposure process of the photoresist is interrupted. Between the control measurements one relies upon the short-term stability of the projection system. In comparison with EUV systems, conventional systems have a relatively high level of short-term stability. The stability requirements for the image position when carrying forward the conventional image position control concept leads to an increase in the mechanical stability requirements in the image position from 1 nm with conventional systems to 0.2 nm with EUV systems over a period of 5 minutes. The thermal expansion of the mechanical base structure of the objective is a main error contribution for the stability of the mirror positions. In order to meet the demanded high requirements of image stability, one is currently taking the approach of using materials with extremely low thermal expansion coefficients for the structure of the projection objective. However, these types of material are extremely cost intensive, sensitive and difficult to process.